The Pali commentaries explain:
One loves all beings:
(a) by the non-harassment of all beings and thus avoids harassment;
(b) by being inoffensive (to all beings) and thus avoids offensiveness;
(c) by not torturing (all beings) and thus avoids torturing;
(d) by the non-destruction (of all life) and thus avoids destructiveness;
(e) by being non-vexing (to all beings) and thus avoids vexing;
(f) by projecting the thought, "May all beings be friendly and not hostile";
(g) by projecting the thought," May all beings be happy and not unhappy";
(h) by projecting the thought, "May all beings enjoy well-being and not be distressed."
In these eight ways one loves all beings; therefore, it is called universal love.
And since one conceives (within) this quality (of love), it is of the mind.
And since this mind is free from all thoughts of ill-will, the aggregate of love,
mind and freedom is defined as universal love leading to freedom of mind.
Extracted from : Metta
The Philosophy and Practice of Universal Love
by Acharya Buddharakkhita